[WTF::HashMap Refactor] Use DeprecatedAtOrEmptyValue() in /third_party/blink/renderer/core/html/track

This CL is a true behavior-preserving refactor. It will not affect
performance because HashMap<>::DeprecatedAtOrEmptyValue() has an
identical implementation to HashMap<>::at().

In the near future, we will change the definition of HashMap<>::at() to
CHECK that the map contains the given key.

All occurrences of DeprecatedAtOrEmptyValue will need to be cleaned up.
If the caller cannot know whether the map already contains the given
key, one possible workaround is to use HashMap<>::find() instead.

Design doc: https://docs.google.com/document/d/18JIiajErikZaBzCtZvl-wwAJShqZsg0jT9wVyPBhSdU/edit?usp=sharing

This CL was uploaded by git cl split.

R=fs@opera.com

Bug: 1058527
Change-Id: I6fd35098c79037358c14dea8edb75480134d6bf2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3082261
Auto-Submit: Dan McArdle <dmcardle@chromium.org>
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Dan McArdle <dmcardle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#909974}